Traditional field-applied coatings for flexible roofs are based upon acrylic, urethane or silicone resins. These coatings are often spray applied over existing or new roofs made from EPDM, asphalt, sprayed foam, polyolefin or other membrane, and they must be highly flexible to resist cracking or other damage due to hail and other environmental stresses. A significant fraction of these coatings are white in color, to reflect a high fraction of solar radiation, and thereby reduce building energy costs by maintaining a cool roof. Some standards for cool roofs are defined by the Cool Roof Ratings Council. However, the coatings of the current art do not often maintain their reflectivity over time, due to two main factors: 1) dirt pickup and environmental staining of the coating, and 2) sunlight/UV degradation and erosion of the polymer binder in the coating.
Reflectivity can also be lost if the topcoat polymer binder has (or develops after weathering) inadequate barrier properties to prevent the leaching to the surface of low molecular weight components, such as plasticizers or colored impurities (e.g. asphaltic materials), which are present in interior layers of the roofing system. These plasticizers and colored impurities serve as a food source for fungi and other biological organisms.
When they are near the surface of the roof, they promote the growth of colored fungi, molds, mildew and algae, which causes staining and further decreases the reflectivity of the roof. The growth of biological organisms is also promoted by high rates of water absorption in the coating layer. Thus, protective roofing coatings are needed which have very low levels of water absorption (initially and after outdoor weathering), and which also block the migration of plasticizers and other colored impurities from the interior of the roofing system.
One method, known in the art to reduce dirt pickup, is to make the topcoat polymer binder “harder”, either by raising the glass transition temperature (Tg) of the polymer, or by increasing the level of crosslinking in the final polymer film to high levels, as described in U.S. Pat. No. 6,680,357 and U.S. Pat. No. 6,833,414. Unfortunately, the ability to do this in practice for flexible roof coatings is limited by the requirement that the coating maintain excellent flexibility. Higher Tg coatings also generally require higher levels of volatile organic solvents (VOC), which can limit their use or disqualify them as ecologically friendly, low VOC coatings. If insufficient levels of coalescing solvents (VOCs) are used with these harder materials, the coatings crack upon application, and no longer have good water repellency or protective properties.
U.S. Pat. No. 5,532,304 likewise teaches a method of combining crosslinkable fluoropolymers with certain curing agents and also tetrafunctional hydrolyzable silane oligomers, in order to increase the stain resistance of the coating. However, this patent does not concern itself with maintaining the flexibility of the coating, in order to maintain a favorable balance of properties over flexible substrates. Moreover, in this reference, high levels of tetrafunctional silane oligomers are preferred (from 5 to 50% based on fluoropolymer), which will greatly limit the flexibility of the coating.
Highly weatherable “cool roof” paints for roofing, made using coil coatings of solvent dispersions of fluoropolymers including poly(vinylidene fluoride) resins, are known, for instance KYNAR 500® PVDF resin (Arkema Inc.), or solvent solutions of fluoropolymers such as poly(vinylidene fluoride) copolymers such as KYNAR 9301 (Arkema Inc.), Zeffle LC 700 (Daikin), so-called FEVE resins (e.g. Lumiflon resins from Asahi Glass), and perfluoroether coatings as taught in U.S. Pat. No. 6,242,557. Many of these paints show excellent stain and dirt resistance, and maintain their “cool roof” characteristics over a long period of time, but they use high levels of VOCs and so do not qualify as low VOC coatings. Moreover, the coil coatings require high baking temperatures, and therefore they cannot be applied on most flexible roofing substrates. They likewise cannot be site-applied on existing roofs, for the same reason.
Surprisingly, it has now been found that a coating composition containing an aqueous-based, acrylic-modified fluoropolymer can be used on flexible substrates to provide significantly improved durability, improved stain repellency including the ability to block plasticizer migration, low water absorption, lower dirt pickup and longer retention of reflectivity, coupled with low levels of VOCs.